High voltage cascaded network circuit

Abstract: A high current, high voltage transistor which can be easily electrically stacked to to extend the voltage range and uses less silicon area than a conventional stacked transistor configuration and a configuration of field plates that provide the greatest breakdown voltages with the highest ohmic values. Also, a star shaped field plate design which provides the greatest breakdown voltages with the highest ohmic values. The field plate is constructed using several concentric rings connected by fingers that are wider at towards the center of the concentric rings and narrower towards the perimeter of the concentric rings.

Description: This invention relates generally to circuits comprising high voltage transistors such as a high voltage electrically stackable NMOS transistor which is fabricated with a "star" shaped field plate that is also a high value high voltage resistor which facilitates electrical stacking of several transistors and thus extend the voltage range for the circuit. Configurations of field plates that provide the greatest breakdown voltages with the highest ohmic values are also disclosed. NMOS transistors are lateral devices. In high voltage applications, the transistor is designed to withstand high voltages by constructing a field plate between the gate and drain regions . The field plates space out the equipotential field lines to avoid voltage concentrations. To increase the voltage range of the circuit, it is common to stack several devices in a series configuration. However, when electrically stacking several transistors together, high voltage resistors are needed to set the correct bias potentials at the gates of the several devices. In discrete forms, high voltage components, including high voltage resistors, are very bulky. Even in integrated forms, high voltage resistors are very large. Therefore a large area of silicon is needed to accommodate the high voltage resistors when integrated with high voltage transistors. High voltage devices can be conventionally integrated using well know integrated device processes. Figure 1 shows a planar view looking down on a conventional high voltage NMOS device 10. While the preferred embodiment of this device is circular, any annular configuration may be used, for example ellipses. However, it is preferable to avoid geometries with sharp or angular corners. The center of the conventional high voltage NMOS device 10 is a drain area 12. Adjacent to and outside of the drain area 12 is a spiral field plate 14. The spiral field plate 14 is made from polysilicon, which has a high resistance. Adjacent to and outside of the spiral field plate 14 is a gate 16. Adjacent to and outside of the gate 16 is a source area 18. Providing an electronic connection between the drain area 12 and the spiral field plate 14 is a metal drain/field plate connector 13. Providing a connection between the spiral field plate 14 and the gate area 17 is a metal gate/field plate connector 15. Figure 2 shows a partial cross-section of the high voltage NMOS device 10. Only a partial cross-section is shown since the device is symmetrical across centerline C1. The drain area 12 surrounds the centerline C1. Outside of and adjacent to the drain area 12 is the spiral field plate 14. Providing an electrical connection between the spiral field plate 14 and the drain area 12 is the drain/field plate connector 13. Outside of the spiral field plate 14 is the gate area 17. Providing an electrical connection between the spiral field plate 14 and the gate 16 is the gate/field plate connector 15. Outside of and adjacent to the gate area 17 is the source area 18. Figure 3 shows a schematic of the conventional high voltage NMOS device 10. Identical elements are labeled with the same identifying numbers used in Figure 1.The drain area 12, gate area 16, and source area 18 are the basic components of the conventional high voltage NMOS device 10. The spiral field plate 14 is represented as a resistor connected between the drain area 12 and the gate 16. A similar device is known from FR-A-2 144 741. Alternatively, the spiral field plate 14 could be connected between the drain area 12 and the source area 18 as shown in Figure 4. A special high voltage MOS device is described in US-A-4 288 806. It comprises source, drain and gate regions with corresponding terminals. Connected to the gate and its terminal is a resistor with a corresponding terminal. This device, therefore, is a four terminal device. In some high voltage applications, conventional high voltage NMOS devices 10 are cascaded to extend the voltage range of the resultant device. Such a cascaded electrical series network is shown in Figure 5 and contains four conventional high voltage NMOS devices 20, 22, 24, 26, although it could be comprised of any number of conventional high voltage NMOS devices. The four high voltage NMOS devices 20, 22, 24, 26 are connected in parallel across a voltage divider network comprised of four resistors 28, 30, 32, 33. Resistor 28 is connected between the gate terminals 34, 36 of high voltage NMOS device 20 and high voltage NMOS device 22. Resistor 30 is connected between the gate terminals 36, 38 of high voltage NMOS device 22 and high voltage NMOS device 24. Resistor 32 is connected between the gate terminals 38, 40 of high voltage NMOS device 24 and high voltage NMOS device 26. Resistor 33 is connected between the gate terminal of high voltage NMOS device 20 and the drain of high voltage NMOS device 20. In order to function properly, the resistors 28, 30, 32, 33 must be constructed as large, high voltage resistors which consume a large part of silicon wafer real estate. Accordingly, it is the primary aim of the invention to provide a circuit comprising a high current, high voltage transistor which can be easily electrically stacked to extend the voltage range of the resultant circuit and uses less silicon area than a conventional electrically stacked transistor configuration. Further advantages of the invention will become apparent as the following description proceeds. The above aims and advantages are achieved with a high voltage cascaded network circuit having the features of claim 1. Figure 1 is a planar view of a conventional high voltage NMOS transistor. Figure 2 is a partial cross section of the conventional high voltage NMOS transistor shown in Figure 1 taken through partial cross-sectionline 2-2. Figure 3 is a schematic of a the conventional high voltage NMOS device shown in Figure 1. Figure 4 is an alternate schematic for the conventional high voltage NMOS device shown in Figure 1. Figure 5 is a schematic of several conventional high voltage NMOS devices shown in Figure 3 in a cascaded network. Figure 6 is a high voltage NMOS device for use in a cascaded network according to the present invention. Figure 7 is a schematic of several high voltage NMOS devices shown in Figure 6 in a cascaded network according to the present invention. Figure 8 is a planar view of a high voltage NMOS device shown in Figure 6 using a spiral resistor. Figure 9 is an alternate planar view of a high voltage NMOS device shown in Figure 6 using a resistor comprised of concentric rings. Figure 10 is an alternate planar view of a high voltage NMOS device using a star shaped resistor network. Alpha-Numeric List of Elements c1 centerline 10 high voltage NMOS device 12 drain area 13 drain/field plate connector 14 spiral field plate 15 gate/field plate connector 16 gate 17 gate area 18 source area 20 high voltage NMOS device 22 high voltage NMOS device 24 high voltage NMOS device 26 high voltage NMOS device 28 resistor 30 resistor 32 resistor 33 resistor 34 gate terminals 36 gate terminals 38 gate terminals 40 a gate terminals 50 high voltage NMOS device 52 gate 53 gate terminal 54 drain 55 drain terminal 56 source 57 source terminal 58 resistor 59 gate/field plate connector 60 resistor terminal 62 cascaded network 64 high voltage NMOS device 66 high voltage NMOS device 68 high voltage NMOS device 70 high voltage NMOS device 72 node 74 node 76 node 78 node 80 node 82 node 84 node 86 resistor 88 resistor 90 resistor 92 resistor 100 high voltage NMOS device 102 field plate 104 spaced concentric rings 106 radial connecting bands 108 drain 110 pad 112 connection line 114 special high voltage NMOS device 116 field plate 118 fingers 120 rings 122 wide inward end 124 narrow outward end 162 metal line Figure 6 shows a schematic of a high voltage NMOS device 50 designed to be used in a cascaded network. It is similar to the high voltage NMOS device 10 shown in Figure 3. It has a gate 52 with a gate terminal 53, drain 54 with a drain terminal 55, and a source 56 with a source terminal 57. Connected to the gate 52 and the gate terminal 53 is a large resistor 58. Unlike the high voltage NMOS device 10 which is a three terminal device, the high voltage NMOS device 50 is a four terminal device. The fourth terminal is a resistor terminal 60 from the resistor 58. The resistor 58 will perform two functions:1) to provide a field plate for the transistor, and 2) as part of a resistor divider network to set up the correct bias voltage for another transistor. The size of the resistor 58 is between approximately 150 mega ohms and 10 giga ohms. Figure 7 shows a cascaded network 62 constructed solely using several high voltage NMOS devices 64, 66, 68, 70. While this cascaded network is shown utilizing four high voltage NMOS devices, a cascaded network could be built utilizing any number of high voltage NMOS devices. High voltage NMOS device 64 is connected to high voltage NMOS device 66 at node 74, which connects the source of high voltage NMOS device 64 with the drain of high voltage NMOS device 66. High voltage NMOS device 66 is connected to high voltage NMOS device 68 at node 76 which connects the source of high voltage NMOS device 66 with the drain of high voltage NMOS device 68. High voltage NMOS device 68 is connected to high voltage NMOS device 70 at node 78 which connects the source of high voltage NMOS device 68 with the drain of high voltage NMOS device 70. The source of high voltage NMOS device 70 is attached to the negative node for the circuit. Resistor 86 of high voltage NMOS device 70 is connected to the gate of high voltage NMOS device 68 at node 80. It is this resistor connection that allows the network to be easily and efficiently cascaded since the resistor can now perform two functions simultaneously. Resistor 86 is now connected between the gates of high voltage NMOS device 68 and high voltage NMOS device 70 performing a voltage divider function between the two high voltage NMOS devices. However, since this is a high voltage application, the high voltage NMOS device 70 also needs a field plate. In NMOS technology, the field plate is connected between the gate and the drain or the source and the drain, as shown earlier in Figure 3 and 4, since there is a large voltage difference between the gate and the drain and only a very small difference between the gate and the source. This high voltage NMOS device 50 can take advantage of the small voltage difference between the gate and the source. In a traditional configuration, the resistor 86 should be connected to the drain of high voltage NMOS device 70 at node 78. However, node 78 is also connected to the source of high voltage NMOS device 68. Since these are high voltage transistors, the difference between the gate of high voltage NMOS device 68 at node 80 and the source of high voltage NMOS device 68 at node 78 is small and can be ignored. Therefore, when the resistor 86 is connected to node 80 instead of node 78, it is still properly biased to perform its function as a field plate for high voltage NMOS device 70. The cascaded electrical series network 62 is thus simplified and space is saved because resistor 86 performs two functions. The first function is as a voltage divider between the gates of high voltage NMOS device 68 and high voltage NMOS device 70. The second function is as a field plate for high voltage NMOS device 70. The rest of the circuit is connected in a similar way. Resistor 88 is connected between the gates of high voltage NMOS device 68 and special high voltage NMOS device 66 at node 80 and node 82 respectively. Resistor 88 performs the voltage divider function between the gates of high voltage NMOS device 68 and high voltage NMOS device 66 as well as the field plate function for high voltage NMOS device 68. Resistor 90 is connected between the gates of high voltage NMOS device 66 and high voltage NMOS device 64 at node 82 and node 84 respectively. Resistor 90 performs the voltage divider function between the gates of high voltage NMOS device 66 and high voltage NMOS device 64 as well as the field plate function for high voltage NMOS device 66. Since this is the last transistor, resistor 92 of high voltage NMOS device 64 is connected back to the drain of high voltage NMOS device 64 at node 72, which is a positive node, as done in the conventional manner. As the high voltage NMOS device 50 is a four terminal device, it is important to remember to connect the resistor of the last transistor to that transistors drain so that the resistor can perform its function as a field plate. In the example shown in Figure 7, four high voltage NMOS devices 64, 66, 68, 70 were cascaded to create the cascaded electrical series network 62. It is important to note, that in creating the cascaded electrical series network 62 that any n + 1, where n is an integer of at least 1, number of high voltage NMOS devices 50 could be used depending on the requirements of the resultant cascaded electrical series network 62. Four high voltage NMOS devices 64, 66, 68, 70 were used merely to illustrate the principles behind cascading high voltage NMOS devices 50 to create a cascaded electrical series network 62. Figure 8 shows a planar view of the high voltage NMOS device 50 shown in Figure 6. Since conventional processes were used to create the high voltage NMOS device 50 attention will be focused on the layout of the high voltage NMOS device 50. The high voltage NMOS device 50 is similar to the high voltage NMOS device 10 in many respects. The innermost region is the drain 54. Outside of the drain 54 is the resistor 58 which will serve two functions, as a field plate and as part of a voltage divider network when the high voltage NMOS devices 50 are cascaded. Outside of the resistor 58 is the gate 52. Outside of the gate 52 is the source 56. There is a gate/field plate connector 59 to provide an electrical connection between the gate 52 and the resistor 58. However, the drain/field plate connector 13 shown in the high voltage NMOS device 10 in Figure 1 has been replaced with with a pad 60 and metal line 162 to provide access to the resistor 58. There is no connection between the drain 54 and the resistor 58 in the high voltage NMOS device 50. The resistor 58 must be connected to either the gate 52 of another high voltage NMOS device 50 in a cascaded fashion or, in the case of the terminating transistor of a cascaded electrical series network, back to the high voltage NMOS device's 50 own drain 54 in order for the transistor to work properly. The regions occupied by the drain 54, gate 52, source 56, and resistor 58 are shown as annular concentric regions but could easily be ovals or some other shape as long as the regions surround each in a concentric fashion. While the previous examples have focused on NMOS technology, the concept of using a resistor in the dual role of both transistor field plate and as part of a voltage divider network of cascaded devices is applicable to both PMOS and bipolar technology. The technique of using a field plate as a large resistor with a dual function, is applicable to high voltage transistors with field plate configurations other than spirals. Figure 9 shows another high voltage NMOS device 100 with another common field plate 102 construction, that of joined concentric rings. The field plate 102 is comprised of several spaced concentric rings 104 connected by two radial connecting bands 106. A drain 108 is located inside and spaced apart from the innermost of the spaced concentric rings 104. There is no electrical connection between the drain 108 and the field plate 102. A pad 110 and metal connection line 112 provide a connecting point to the innermost of the spaced concentric rings 104. The rest of the high voltage NMOS device 100 is constructed in a conventional manner and is not shown. Both spiral field plates and concentric ring field plates have their own intrinsic advantages and disadvantages. For example, high voltage devices made with spiral field plates, as shown in Figures 1 and 8, have a greater break down voltage. However fabrication is easier and it is possible to make resistors with larger ohmic values using the concentric ring method as shown in Figure 9. It would be desirable to find a field plate construction that combines the advantages of spiral field plates and concentric ring field plates. Figure 10, shows a portion of a high voltage NMOS device 114 with a field plate 116. The field plate 116 comprises fingers 118 and rings 120, resulting in a "star" construction. In comparing Figure 10 with Figure 9, the radial connecting bands 106 used in Figure 9 are replaced by fingers 118 used in Figure 10. Focusing on the fingers 118, the fingers 118 have a wide inward end 122 and a narrow outward end 124. Designing a field plate using a series of concentric rings 120 connected with radial fingers 118 facilitates a field plate with the advantages of both spiral field plates, as shown in Figure 8 and ring field plates, as shown in Figure 9. The wide inward end 122 of the fingers 118 provides a physically larger connection between the fingers 118 and the rings 120 where the voltage concentration is the largest. This allows the field plate 116 to more effectively space the equipotential lines. The narrow outward end 124 of the fingers 118 provides a physically smaller connection where the voltage concentration is the smallest. For most efficient operation, the ratio of the width of the wide inward end 122 to the width of the narrow outward end 124 should be approximately 2:1. The rings 120 can be constructed out of a medium resistance polysilicon, typically having values of 10 kilo ohms per square. The rings 120 are fabricated using the same techniques and materials used to produce the spaced concentric rings 104 shown in the high voltage NMOS device 100 in Figure 9. It is this feature which facilitates production of very high ohmic resistors. Typical values range from 150 mega ohms to 10 giga ohms. The fingers are constrcuted of a high resistance polysilicon, typically having values of 2 gigia ohms per square. The number of fingers 118 needed is dependent upon the size of the resistor desired. A very large resistor of approximately 10 giga ohms would only need 1 finger 118. The number of fingers increases inversely with resistor size. A much smaller resistor of 150 mega ohms would need 50 fingers 118. If more than one finger 118 is needed they should be approximately evenly spaced around the circumference of the rings 120. Figure 10 shows a configuration including 7 fingers 118 for illustration purposes only. While a field plate constructed in either a ring structure, as shown in Figure 9, a spiral structure, as shown in Figure 8, or a star structure, as shown in Figure 10, can be used to construct the high voltage NMOS device 50, the star structure shown in Figure 10 has provided the best results because the unique design of the star structure combines very large resistor characteristics of the traditional concentric ring design with the higher breakdown voltages found in spiral designs.

Claims: A high voltage cascaded network circuit comprising: a) an electrical circuit (62) of at least two four-terminal high voltage transistors (50,64,66,68,70,100,114) connected in a cascade configuration from a first transistor (70) to a last transistor (64), each transistor comprising: i) a source region, a drain region, a gate region, and a resistor region, ii) all of said regions being arranged in a concentric configuration with said drain region being the innermost region, said resistor region surrounding said drain region, said gate region surrounding said resistor region and said source region surrounding said gate region, iii) a source (56) having a source terminal (57), said source being located in said source region, iv) a drain (54,108) having a drain terminal (55), said drain being located in said drain region, v) a gate (52) having a gate terminal (53), said gate being located in said gate region, vi) a resistor terminal (60), and vii) a resistor means (58,86,88,90,92) having two ends, said resistor means being located in said resistor region, one end of said resistor means being electrically connected to said gate terminal (53) and the other end of said resistor means being electrically connected to said resistor terminal (60), said resistor means forming a field plate electrode for the transistor, said high voltage cascaded network circuit further comprising: b) a positive node (72), c) a negative node, d) the source terminal of said first transistor (70) being connected to said negative node, e) the resistor terminal and the drain terminal of the drain of said last transistor (64) being connected to said positive node (72), and f) each of said transistors, other than said last transistor (64) having: i) its drain terminal electrically connected to the source terminal of the succeeding transistor in said cascade (62), ii) its resistor terminal electrically connected to the gate terminal of the succeeding transistor in said cascade (62), g) each of said transistor, other than said first transistor (70) having: i) its source terminal electrically connected to the drain terminal of its preceding transistor in said cascade (62), and ii) its gate terminal electrically connected to the resistor terminal of its preceding transistor in said cascade (62).

Inventors: MOJARADI MOHAMAD M, VO TUAN A, MOJARADI, MOHAMAD M., VO, TUAN A.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1994
Main classification: H01L29
IPC codes: H01L29, H03K17, H01L21, H01L23, H01L27